The present invention relates generally to gypsum wallboard construction material, and more particularly to the modification of such materials by the addition of silica-based geopolymer adhesives to produce composites which exhibit improved fire performance, water resistance, and structural properties.
Gypsum wallboard is a widely used construction material because of its low cost and fire resistance. Fire resistance capability is generally proportional to the thickness of the gypsum employed in a fire resisting structure. For example, a simple structure approved for a fire endurance period of one hour uses a 5/8 in. thick slab of Type X gypsum wallboard on either side of a 35/8 in. metal stud with an air filled cavity for a non-load-bearing wall assembly. Other designs provide fire resistance for periods of up to two hours.
Gypsum is a naturally occurring form of the di-hydrate of calcium sulfate. This material can be readily transformed to its stucco form, the hemi-hydrate of calcium sulfate, by one of several calcination processes. Gypsum provides fire protection through two primary mechanisms: the non-combustible nature of inorganic compounds; and the endothermic, energy-absorbing capacity of the dihydrate which produces steam when exposed to intense heat. In equation form, gypsum.fwdarw.Plaster of Paris+Steam-Energy, or CaSO.sub.4 (2H.sub.2 O).fwdarw.CaSO.sub.4 (0.5H.sub.2 O)+1.5H.sub.2 O-4100 cal/mole.
A wall which is to endure fire for a period of at least one hour must be able to withstand temperatures well in excess of 1500.degree. F. This temperature is considerably in excess of the ignition temperature of most organic materials (about 450.degree.-800.degree. F.). Thus, a fire-resistant wall must maintain the temperature of the unexposed face of the wall at a moderately low temperature in order to prevent the spread of the fire. A further consideration for an acceptable fire-resistant wall is the integrity of the wall assembly against penetration by a water stream from a fire hose at the termination of the fire exposure period. Masonry walls achieve these conditions as a result of the high structural/thermal mass and large heat capacity inherent in the dense materials utilized. Lightweight wall assemblies, by contrast, require good thermal insulation in lieu of large thermal mass. The utilization of the heat of dehydration of gypsum can provide an effective cooling mechanism for a fire-resistant wall. However, as this water of hydration, which binds the gypsum material, is converted to steam, the gypsum is recalcined into a fine hemi-hydrate powder, or stucco, leaving a wall component that is devoid of structural integrity, lacking dimensional stability, and without strength after fire exposure. Moreover, the recalcined gypsum material is easily washed away with water from a fire hose. At a minimum, the wall is easily crumbled by the action of water from fire hoses because of shrinking and bending of the studs as well as from the shrinking of the gypsum itself which produces cracks therein.
Structural rigidity of gypsum wallboard, which is proportional to the moment of inertia of the paper facing sheets about the bending axis, derives from the bond of the paper to the core. This bond is affected by the degree of saturation of the paper which also promotes rehydration and crystalline growth of the gypsum into the paper. In addition, the compressive strength of the gypsum core is proportional to the density, the type of crystallization, and the degree of rehydration. Of the naturally occurring forms of gypsum, three are common. Acicular or needle-like satinspar and plate-like selenite do not have adequate structural integrity to be of interest in the construction trades, and physical conditions which promote the growth of these forms prevent the formation of structural gypsum. Massive gypsum, or alabaster, has random three-dimensional crystalline orientation, and while not highly soluble in water, it is hygroscopic and will soften when wet. During this condition, it loses most of its strength and in a wallboard structure, the gypsum/paper crystalline bond is easily destroyed or damaged. If properly dried, the core strength will return.
Care must be exercised in the selection of additives for gypsum wallboard, since a poor choice of additives may cause improper recrystallization (development of an interpenetrating network through random crystalline growth), with a resulting loss of strength both in the core and in the paper facing sheet bond. In addition, the presence of soluble salts and other impurities affects the gypsum/paper bond and the core strength detrimentally. Finally, the setting time, which is directly related to the rate of rehydration and release of heat, must be maintained within strict limits to meet operational criteria of a wallboard manufacturing process specification; that is, the mix must retain a working viscosity while in the mixer and during the wetting of the paper facing sheets, must be plastic but hold its form while passing through the forming rolls and smoothing bars, and must cure to moderate handling strength sufficient for cutting and transport to the dryer within the length of a production line which is typically 4-5 min. for a 1000 ft. line and a board movement rate of 200-250 ft./min.
Retarders are used to prevent initial setting during the first phase and accelerators are used to produce final set within the proper time period. Different compounds affect the initial set and rehydration cycle and the chemistry becomes complex and highly proprietary. The primary accelerator is the addition of freshly ground massive gypsum as a seed crystal. Without retardation, typically by using water-soluble organic acids or bases, the nucleation will proceed to rapid initial set, thereby preventing proper wetting and forming. Additional accelerators such as potash, for example, are employed to enhance the rehydration phase to produce "Vicat Set." That is, the point where the strength measured by a Vicat hardness tester is adequate for cutting and drying, at which time the rehydration exotherm has reached completion and the temperature of the material remains approximately constant.
Natural massive gypsum has a density of about 2.35 g/cm.sup.3 (147 lb/ft..sup.3) which is approximately three times the density of typical fire-resistant gypsum wallboard (47 lb/ft..sup.3). Reconstitution of massive gypsum di-hydrate at atmospheric pressure will not produce the high density of the natural gypsum because of the requirement of excess water to generate a workable slurry mixture. The cured density of Plaster of Paris ranges from a high of about 75 lb/ft..sup.3 with a water:stucco wet weight ratio of 0.8 to a low of 36 lb/ft..sup.3 with a corresponding ratio of 1.7. Compressive strength falls from 2000 psi to 170 psi, respectively. The exact formula specification of 1.5 moles of water added to one mole of calcium sulfate hemi-hydrate to produce the massive gypsum corresponds to a water:stucco ratio of 0.186. Generally, the amount of available water directly determines the final cured gypsum density.
Accordingly, an object of the present invention is to provide a gypsum-based construction material having improved dimensional and structural stability when exposed to fire.
Another object of this invention is to provide a gypsum-based construction material having improved resistance to water after being exposed to fire.
Yet another object of the present invention is to provide a gypsum-based construction material derived from less expensive materials than those currently employed.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.